Additive-controlled asymmetric iodocyclization enables enantioselective access to both α- and β-nucleosides

β-Nucleosides and their analogs are dominant clinically-used antiviral and antitumor drugs. α-Nucleosides, the anomers of β-nucleosides, exist in nature and have significant potential as drugs or drug carriers. Currently, the most widely used methods for synthesizing β- and α-nucleosides are via N-glycosylation and pentose aminooxazoline, respectively. However, the stereoselectivities of both methods highly depend on the assisting group at the C2’ position. Herein, we report an additive-controlled stereodivergent iodocyclization method for the selective synthesis of α- or β-nucleosides. The stereoselectivity at the anomeric carbon is controlled by the additive (NaI for β-nucleosides; PPh3S for α-nucleosides). A series of β- and α-nucleosides are prepared in high yields (up to 95%) and stereoselectivities (β:α up to 66:1, α:β up to 70:1). Notably, the introduced iodine at the C2’ position of the nucleoside is readily functionalized, leading to multiple structurally diverse nucleoside analogs, including stavudine, an FDA-approved anti-HIV agent, and molnupiravir, an FDA-approved anti-SARS-CoV-2 agent.

The authors report an unprecedented additive-controlled iodocyclization approach for the stereoselective synthesis of α-or β-nucleosides. With NaI as additive, β-nucleosides were obtained in high yields and stereoselectivities, whereas addition of PPh3S enabled highly stereoselective synthesis of α-nucleosides. By further functionalization of C2′ iodine-modified nucleosides, synthesis of a range of diverse nucleoside analogues including stavudine was completed. The manuscript is well prepared, and the experimental procures and characterization data were given in detail. Although the preparation of the iodocyclization substrates is a bit lengthy, the approach provides a new avenue to the stereocontrolled synthesis of nucleoside analogues. The significance and novelty of this work could arouse broad interest in this field. I would suggest to address the following issues before publication.
The β-nucleosides were actually very convenient to be obtained by participating group effect at the C2′ position of the sugar moieties in very few steps compared with the present approach utilizing iodocyclization substrates. Could the authors compare the overall steps and yields of these approaches?
The solvents played a significant effect on the stereoselective synthesis of nucleosides. Could the authors provide explanations on the influence of the solvents on the selectivities of the iodocyclization?
How did the additive NaI function in the NIS-promoted cyclization? Could the authors point out the specific function of the various iodine sources?
Why were the high ee values obtained by NaI instead of KI?
In the hydrogen-bonded transition state, where did the iodine come from, NaI or NIS?
In terms of the synthesis of anti-HIV drug stavudine, the advantages of the approach could be presented compared with traditional method. 1. A more concise title could be considered. Perhaps "additive-controlled asymmetric iodocyclization enables enantioselective access to both a-and b-nucleosides" would better summarize the results described in this manuscript. "Nucleoside switches" is awkward in wording and to me implies some type of switch, which is not a topic of the work described here.
2. Mention is made to the importance of facilitating access to anti-SARS-CoV-2 drugs or drug carriers (i.e., all of paragraph 2), and the manuscript reports in the last line of the conclusion (the ultimate summary sentence) that this work contributes to the battle against COVID-19; however, no access to anti-SARS-CoV-2 drugs or drug carriers is presented here or a rationale to how this work contributes to the fight against COVID-19 is offered. To better convey this point, access to anti-SARS-CoV-2 drugs should be added to this research. I believe your work could provide elegant access to molnupiravir, for example. Furthermore, a side-by-side overall step and yield count from readily commercially available starting materials using your method to molnupiravir and the current state-of-the-art would be very helpful. This would certainly increase the interest of the manuscript to the general scientific community.
3. A general scheme or at least mention of the step and yield counts to your starting chiral alcohols in the main text (as opposed to somewhat segmented schemes in the SI) would be beneficial to the reader.
5. While your mechanisms are very likely plausible, some references delineating the rationale of the intermediate species you propose (or some basic computational work) would help.
I add a few minor points: • Stereoselective and protecting group-free access to nucleosides via a 1,2-anhydrosugar has been described and has been used to provide antiviral drugs as well. Pertinent references include Org Lett 2015, 18, 46054 andCEJ, 2017, 23, 3910 and should be cited.
• Page 2, line 41 -COVID-19 stands for coronavirus disease 2019 • Page 2, line 42 -"Experienced" is a better word than "witnessed" everyone has experienced the pandemic in one capacity or another.
• In the SI, in Scheme S1, you have CHCl3 above the second arrow; I believe you mean CHI3.
• The overall quality of English is commendable, only small errors persist. At this time, I believe they are not worth mentioning as the text does require a thorough revision, so the syntactical errors can be addressed in the next iteration.
Reviewer #3 (Remarks to the Author): This manuscript describes a chiral phosphoric acid catalyzed halo-oxycyclization between ene-urea and NIS. Various substituted 3-tetrahydrofurans can be accessed in high yields with excellent enantioselectivity and diastereoselectivity. Representative products have been transformed into biologically active nucleoside analogues. This paper is written as a methods paper focused on synthetic utility. That's why very specific 4-aminobut-3-en-1-ol derivatives were selected to undergo cyclization, so the scope of the reaction is quite limited. A stereochemical model is provided, however it is entirely speculative and does not add meaningfully to the paper.
Despite this merit, however, the present study is not impressive in terms of novelty. As mentioned by the authors, this strategy was previously reported by Trost et al (see J. Am. Chem. Soc. 2019, 141, 10199−10204) via Pd-catalyzed enantioselective iodoetherification. The authors reported the same enantioselective transformations that can also be carried out using chiral phosphoric acid catalysts. Compared with the reported protocols, the present one is not superior in terms of catalytic efficiency, while an interesting stereodivergent iodocyclization have been described. This precedent work thus reduces the novelty and scientific impact of this contribution.

Reviewer 1
The authors report an unprecedented additive-controlled iodocyclization approach for the stereoselective synthesis of αor β-nucleosides. With NaI as additive, β-nucleosides were obtained in high yields and stereoselectivities, whereas addition of PPh3S enabled highly stereoselective synthesis of -nucleosides. By further functionalization of C2′ iodine-modified nucleosides, synthesis of a range of diverse nucleoside analogues including stavudine was completed. The manuscript is well prepared, and the experimental procures and characterization data were given in detail. Although the preparation of the iodocyclization substrates is a bit lengthy, the approach provides a new avenue to the stereocontrolled synthesis of nucleoside analogues. The significance and novelty of this work could arouse broad interest in this field. I would suggest to address the following issues before publication.
-The -nucleosides were actually very convenient to be obtained by participating group effect at the C2′ position of the sugar moieties in very few steps compared with the present approach utilizing iodocyclization substrates.
Could the authors compare the overall steps and yields of these approaches?
Response 1: We thank the reviewer for the suggestions. As the reviewer said, the widely applied Vorbrüggen Nglycosylation utilizes the participating group effect of the C2' participating group to achieve  selectivity. It uses a strong lewis acid to catalyze coupling between a per-acetylated sugar synthon and a per-trimethylsilylated nucleobase via a 1', 2'-dioxolenium ion intermediate. The α-face of the molecule is blocked to nucleophilic attack, which results in high ꞵ-selectivity. The protecting groups of sugar ring, especially the group at the C2' position, determines the stereoselectivity at the anomeric C1' carbon in these methods. Therefore, along with other newly developed methods, including Yu glycosylation, it is substrate dependent. Otherwise, the method we developed is an additive-controlled asymmetric method, which is unprecedented in nucleoside synthesis, to the best of our knowledge, to synthesize and -nucleosides from even the same precursor, with satisfying yields and diastereoselectivities. We believe our method is of great value, especially for the medicinal chemistry.
Here, we take the synthesis of stavudine, anti-HIV agent, as an example to compare our approach with the reported methods. The reported syntheses of stavudine, commencing with D-ribose, are of 6 to 10 overall steps with 17-60% overall yields (Scheme R1). In our work, we utilized the NaI-additive iodocyclization as the key step to develop an efficient route to synthesize stavudine (7 steps, 43% overall yield), starting from S-trityl glycidyl ether.
We would not claim our method as a game changer, in terms of synthetic efficiency and simplicity. But we would like to humbly point out the versatility of our additive-controlled method and its potential usefulness for medicinal chemistry. Also, the novelty of the unprecedented additive-controlled stereodivergent iodocyclization method for nucleoside synthesis should be emphasized. Scheme R1. Chemical routes to stavudine synthesis.
-The solvents played a significant effect on the stereoselective synthesis of nucleosides. Could the authors provide explanations on the influence of the solvents on the selectivities of the iodocyclization?

Response 2:
We thank the reviewer for the suggestions. The solvents do affect the stereoselectivities in our method (PhMe and CHCl3 beneficial for R-selectivity, CH2Cl2 beneficial for S-selectivity). As suggested in the computational mechanistic study (Figure 2 in main text), in the R-selective reactions, the additive NaI plays a centered role that cooperates with C1, NIS  Response 3: We thank the reviewer for the question and suggestion. We ran a series of control experiments to explore the mechanism of -selective iodocyclization (see Figure 2 in main text and  Figure   S3 in SI), in a unique manner benefitting stereoselectivity.
In addition, the computational studies elucidate the effect of additive NaI in -selective iodocyclizations. As shown in Figure S2 in SI, the Gibbs energy of reaction to form Int-I (-49.9 Figure S3 in SI (left), blue represents a covalent bond or ionic bond, and green represents weak interaction. NaI is found as a centered role that cooperates with C1, NIS and substrate 1a through LP... interactions and Na-O interactions, providing an excellent stereoselective environment for R-selectivity.

Int-SI (-13.7 kcal/mol), indicating the essence of NaI. We further use interaction region indicator (IRI) and fuzzy bond order (FBO) to analyze the interactions between atoms of Int-I. As shown in
-Why were the high ee values obtained by NaI instead of KI?
Response 4: We thank the reviewer for the question. As shown in Figure S3 in SI, we analyzed the interaction -Minor points: Page 11: "alkyne 20" and "alkyne 21" were not correct. They should be alkenes.
Reference 38: the journal name "PANS" was wrong. Please correct it.

Response 8:
We thank the reviewer for pointing these out. We have corrected these typographical errors.

Reviewer 2
This manuscript by Wang et al. highlights an asymmetric chiral phosphoric acid-catalyzed approach to both and -nucleoside analogues by modifying the additive in good yields, diastereomeric ratios, and enantiomeric excesses. The manuscript is tidy (with only minor syntactical English errors) and no doubt demonstrates an appreciable body of research; at this time, I recommend this manuscript for acceptance with major revisions.
Nature Communications is a preeminent scientific journal for disseminating results of general interest to the broader scientific community; some manipulations to the text and references are necessary to better convey this, and some additional experiments should be run. I clarify below.
-A more concise title could be considered. Perhaps "additive-controlled asymmetric iodocyclization enables enantioselective access to both and -nucleosides" would better summarize the results described in this manuscript. "Nucleoside switches" is awkward in wording and to me implies some type of switch, which is not a topic of the work described here.

Response 1:
We thank the reviewer for the comment and kind suggestion. The title has been changed to 'Additivecontrolled asymmetric iodocyclization enables enantioselective access to both and -nucleosides' in the revised manuscript.
-Mention is made to the importance of facilitating access to anti-SARS-CoV-2 drugs or drug carriers (i.e., all of paragraph 2), and the manuscript reports in the last line of the conclusion (the ultimate summary sentence) that this work contributes to the battle against COVID-19; however, no access to anti-SARS-CoV-2 drugs or drug carriers is presented here or a rationale to how this work contributes to the fight against COVID-19 is offered. To better convey this point, access to anti-SARS-CoV-2 drugs should be added to this research. I believe your work could provide elegant access to molnupiravir, for example. Furthermore, a side-by-side overall step and yield count from readily commercially available starting materials using your method to molnupiravir and the current state-of-the-art would be very helpful. This would certainly increase the interest of the manuscript to the general scientific community. -A general scheme or at least mention of the step and yield counts to your starting chiral alcohols in the main text (as opposed to somewhat segmented schemes in the SI) would be beneficial to the reader.

Response 3:
We thank the reviewer for the helpful suggestion. Since the synthetic route of the chiral alcohol 4 is relatively simple, description of its synthesis was added in the SI (Page S9). Also, it is showed in Scheme 6 in main text regarding the synthesis of two FDA-approved agents stavudine and molnupiravir (Page 16 in the revised manuscript), which is hopefully beneficial to the reader.

Response 4:
We thank the reviewer for the kind comment and suggestion. The C3'-deoxy-α-threopentonucleofuranosides' bioactivities, to the best of our knowledge, have not been explored so far. And the C3'deoxy-α-threo-pentonucleofuranosides have not been synthesized yet. As we mentioned in the introduction part: "The absence of an efficient method for the synthesis of -nucleosides presents a major roadblock for the further exploration of -nucleoside bioactivity". Hopefully, the method we report will present a scalable access to the -nucleosides and faciliate further bioactivity study.
Scheme R4. Chemical routes to 3'-deoxypyrimidine nucleosides syntheses. species you propose (or some basic computational work) would help.

Response 5:
We thank the reviewer for the kind comment and suggestion. To gain a better understanding of the mechanism of our methodology, a series of control experiments (see Table S14 in SI) and computational studies were performed (Figure 2 in main text), which are all included in the revised paper.
I add a few minor points: -Stereoselective and protecting group-free access to nucleosides via a 1,2-anhydrosugar has been described and has been used to provide antiviral drugs as well. Pertinent references include Org Lett 2015, 18, 46054 andCEJ, 2017, 23, 3910 and should be cited.

Response 6:
We thank the reviewer for the helpful suggestions. In accordance with the reviewer's suggestion, stereoselective and protecting group-free access to -nucleosides via 1,2-anhydrosugar has been well developed.
In the revised manuscript, we stated 'In most glycosylation, the sugar synthons are fully protected. Hocek et al.
reported the glycosylation of nucleobases with 5'-O-monoprotected ribose or C5'-modified ribose derivatives using modified Mitsunobu conditions to yield -nucleosides, whose anomeric selectivity highly depends on the hydroxy group at C2' of the ribosyl donor' in page 4. Also, the suggested papers were cited (citation 26 and 27) in the revised manuscript.
-Page 2, line 41 -COVID-19 stands for coronavirus disease 2019 -Page 2, line 42 -"Experienced" is a better word than "witnessed" everyone has experienced the pandemic in one capacity or another.

Response 7:
We thank the reviewer for the helpful comments and kind suggestions. These points have been modified.
-In the SI, in Scheme S1, you have CHCl3 above the second arrow; I believe you mean CHI3.

Response 8:
We thank the reviewer for pointing this out. We have corrected this typographical error.
-The overall quality of English is commendable, only small errors persist. At this time, I believe they are not worth mentioning as the text does require a thorough revision, so the syntactical errors can be addressed in the next iteration.

Response 9:
We thank the reviewer for the kind comment and suggestion. In the modified manuscript, further proofreading has been done, with the modifications highlighted yellow.
Various substituted 3-tetrahydrofurans can be accessed in high yields with excellent enantioselectivity and diastereoselectivity. Representative products have been transformed into biologically active nucleoside analogues. This paper is written as a methods paper focused on synthetic utility. That's why very specific 4aminobut-3-en-1-ol derivatives were selected to undergo cyclization, so the scope of the reaction is quite limited.

Response 1:
We thank the reviewer for the extremely helpful comments. In the current method we report, which utilized stereodivergent iodocyclization to synthesize ribofuranonucleosides, 4-aminobut-3-en-1-ol derivatives were adopted as the precursors. As the reviewer stated, 'various substituted 3-tetrahydrofurans can be accessed in high yields with excellent enantioselectivity and diastereoselectivity' even in a stereodivergent manner. With this paper, we are very excited to report this finding. In addition, the following research underwent in our lab has proved the azanucleosides could also be obtained via similar cyclization strategy.
-A stereochemical model is provided, however it is entirely speculative and does not add meaningfully to the paper.

Response 2:
We thank the reviewer for the comment. To gain a better understanding of the mechanism of our methodology, a series of control experiments (see Table S14 in SI) and computational studies were performed ( Figure 2 in main text), which are all included in the revised paper.
-Despite this merit, however, the present study is not impressive in terms of novelty. As mentioned by the authors, 2) The revised paper includes a series of control experiments (see Table S14 in SI) and computational studies recently performed (Figure 2 in main text), which elucidate the mechanism of our method. It was revealed that NaI cooperates with C1, NIS and the substrate to catalyze R-selective iodocyclization in a unique manner, which is even superior to KI ( Figure S3 in SI). The catalytic effect of NaI in our method is, in our opinion, quite novel and illuminating for further study.
3) Prof. Trost's work basically focused on the synthesis of nucleoside analogs bearing seven-membered sugar rings. Our method can be applied to the synthesis of ribofuranonucleosides with biological effect, even in an additive-controlled stereodivergent manner. We believe this is of great interest to the medicinal chemistry.
-Finally, the method was applied to the synthesis of nucleoside analogs, including stavudine, thymidine and ꞵ-D-ddT. To demonstrate synthetic utility, it would have been interested to compare their syntheses with the previous ones (shorter? better overall yield).

Response 4:
We thank the reviewer for the extremely helpful comments and kind suggestions. Here, we take the synthesis of stavudine, anti-HIV agent, as an example to compare our approach with reported methods. The reported syntheses of the stavudine, commencing with D-ribose, are of 6 to 11 overall steps with 17-60% overall yields (Scheme R5). In our work, we utilized the NaI-additive iodocyclization as the key step to develop an efficient route to synthesize stavudine (7 steps, 43% overall yield), starting from S-trityl glycidyl ether.
We would not claim our method as a game changer, in terms of synthetic efficiency and simplicity. But we would like to humbly point out the versatility of our additive-controlled method and its potential usefulness for medicinal chemistry. Also, the novelty of the unprecedented additive-controlled stereodivergent iodocyclization method for nucleoside synthesis is noteworthy.
Scheme R5. Chemical routes to stavudine synthesis.
-In addition, the authors used quite hazardous / toxic reagents. such as Bu3SNH and OsO4 which can limit industrial applications.

Response 5:
We thank the reviewer for the helpful comments. As the reviewer stated, Bu3SnH and OsO4 are inacceptable in industrial production. Meanwhile, we are constantly working on solving these issues using green chemistry.
-While the reaction is optimized to an outstanding point, the readership of Nature and other top-flight journals expect something more insightful. The catalyst substrate activation method also lacks novel insight as hypothesized. As a result, this work strikes me as rather specialized, and would be a strong communication in Org Lett, Chem Comm, or the equivalent, or potentially JACS as an article (however, more experimental support for the mechanistic hypothesis would be expected). The authors are to be commended for the hard work and persistence that has led to a very nice piece of work. It simply doesn't meet the expected level of impact for a publication in this journal.

Response 6:
We thank the reviewer for the kind comments. We ran a series of control experiments (see Table S14 in SI) and carried out computational studies to demonstrate the reaction mechanism (Figure 2 in main text), which is hopefully helpful to improve the scholar presentation and impact of our work.